An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a current supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the current applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a steering assist command value of an assist (steering assist) command based on a steering torque Tr detected by the torque sensor 10 and a velocity Vel detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on a current control value E obtained by performing compensation and so on with respect to the steering assist command value. Moreover, it is also possible to receive the velocity Vel from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the velocity Vel from the velocity sensor 12 are inputted into a steering assist command value calculating section 101, and a steering assist command value Iref is calculated by means of an assist map. The calculated steering assist command value Iref is inputted into a maximum output limiting section 102 and an output is limited based on an overheat protection condition or the like in the maximum output limiting section 102. A current command value I that maximum output is limited, is inputted into a subtraction section 103.
Moreover, with respect to the calculation of the steering assist command value Iref performed in the steering assist command value calculating section 101, it is also possible to calculate the steering assist command value Iref by using not only the steering torque Tr and the velocity Vel but also a steering angle.
The subtraction section 103 calculates a deviation ΔI(=I−i) between the current command value I and a motor current of the motor 20 that is fed back, the deviation ΔI is controlled by a current control section 104 such as a PI (proportional and integral) or the like, the controlled current control value E is inputted into a PWM control section 105 and the duty ratio is calculated, and in accordance with a PWM signal PS that the duty ratio is calculated, the motor 20 is driven through a motor drive circuit 106. The motor current i of the motor 20 is detected by a motor current detection circuit 107, and the motor current i is inputted into the subtraction section 103 to feed back.
A bridge circuit that bridge-connects semiconductor switching elements (EFTs) and the motor, is used in the motor drive circuit that controls the motor current by means of the current control value E and drives the motor. The motor drive circuit that is configured so as to control the motor current by ON/OFF-controlling the semiconductor switching elements in accordance with the duty ratio of the PWM signal determined based on the current control value E, is widely used. Recently, a brushless DC motor is used as the motor. Hereinafter, a motor drive circuit of a three-phase brushless DC motor will be described in brief.
The brushless DC motor is constructed so that an armature coil is wound around a stator, a rotor is comprised of permanent magnets, and determining the timing of a direct current flowing in a coil of a magnetic pole corresponding to a position of the permanent magnets in accordance with a rotational position of the permanent magnets so that a magnetic field of the permanent magnets is perpendicular to a magnetic field produced by the armature coil. A rotation sensor for detecting the rotational position of the permanent magnets is disposed in the stator, the number of detecting elements of the rotation sensor is proportional to the number of phases of the motor, and in the case of a three-phase brushless DC motor, three detecting elements are required. Inexpensive Hall elements (Hall sensors) or the like are used as the detecting elements of the rotation sensor.
FIG. 3 is a wiring diagram showing a schematic configuration of the motor drive circuit 106 that drives the three-phase brushless DC motor 20. The motor drive circuit 106 is an inverter circuit comprised of six semiconductor switching elements SW1˜SW6, and is comprised of a serial connection of the semiconductor switching elements SW1 and SW2 (A-phase), a serial connection of the semiconductor switching elements SW3 and SW4 (B-phase) and a serial connection of the semiconductor switching elements SW5 and SW6 (C-phase). Electric power is supplied to the motor drive circuit 106 from the battery 13. Further, the rotation sensor for detecting electrical angle of the motor 20 is comprised of three detecting elements (Hall elements) H1, H2 and H3, the detecting elements H1˜H3 are disposed so that a phase difference between a neutral axis of each phase and each detecting element becomes 60°, and phase current values I1, I2 and I3 of three phases are ON/OFF-controlled in accordance with rises and falls of the detecting elements H1, H2 and H3 that detect the electrical angle of the motor 20.
FIG. 4 shows timings of rises and falls of the detecting elements H1˜H3 as the rotation sensor and timings of ON/OFF-control of the switching elements SW1˜SW6. As shown in FIG. 4, in the rise of the detecting element H1, the switching element SW1 becomes an ON state, and in the rise of the detecting element H2, the switching element SW1 becomes an OFF state, thus A-phase is excited. The switching element SW4 becomes an ON state from the rise of the detecting element H1 to rotating 60°, and the switching element SW6 becomes an ON state after rotating 60°, thus B-phase and C-phase are excited so as to become reverse polarity each other. In this way, since two phases are excited simultaneously, it is possible to effectively drive the motor 20. In order to reverse the motor 20, just need to reverse the relationship of ON/OFF-control of the switching elements SW1˜SW6 corresponding to rises and falls of the detecting elements H1˜H3.
In a case that an insulation breakdown occurs in the switching elements SW1˜SW6 of such the motor drive circuit 106, there is a problem that an abnormal current flows in the motor 20, the motor 20 burns out and the motor 20 acts as an electromagnetic brake. Therefore, a motor relay that lets motor terminals out of the switching elements is inserted into the motor drive circuit 106.
However, when installing the motor relay, extraneous substances attach to relay contacts, the relay contacts are covered by oxidized coating and have a loose connection, even the motor relay is excited, there is a possibility that a failure that the relay contacts become an open state occurs. Further, in a case that the motor current detection circuit broke down, it is impossible to detect a correct motor current, and a proper current control value cannot be outputted, as a result, an inexpedience that an excessive current flows in the motor and an excessive steering assist torque is supplied, or a necessary current does not flow in the motor and a sufficient steering assist torque cannot be supplied, occurs.
In order to avoid an unexpected situation due to such a failure, an initial diagnosis function is installed into the control unit. In starting an engine, the initial diagnosis function is performed, by forcibly applying the motor current, actions of the motor current detection circuit and the relay contacts of the motor relay are confirmed (for example, Japanese Published Unexamined Patent Application No. H8-91239 A (Patent Document 1)).
In the initial diagnosis function, when an open failure of the relay contacts occurs, since an abnormal status similar to a case that a source fault or a ground fault of the motor circuit occurs, based on the abnormal status occurred, the occurrence of the open failure of the relay contacts is presumed. However, it is often the case that the open failure of the relay contacts lack repeatability, and a failure analysis is accompanied by a difficulty. Further, in the detection of the open failure of the relay contacts, if it is possible to identify which relay caused the open failure, the failure analysis can be more quickly performed. However, in the conventional diagnosis function, it is impossible to identify which relay caused the open failure of the relay contacts.
On the other hand, although a matter that insulting extraneous substances attach to the relay contacts is considered as one of the causes of the open failure of the relay contacts, it is empirically known that in a use status that an inrush current (i.e. a current flowing at a moment that the relay contact is closed) flows and the relay contact is closed, the open failure of the relay contact does not occur, meanwhile, in a use state that a current does not flow at the moment that the relay contact is closed, the open failure of the relay contacts occurs at a constant frequency. It is presumed that this is because in the use status that the inrush current flows and the relay contact is closed, the extraneous substances attached to the relay contacts are removed by the inrush current, whereas, in the use state that the current does not flow at the moment that the relay contact is closed and a current flows in the relay contact which is in a closed status, the extraneous substance removal by current is impossible. Therefore, by removing the extraneous substances attached to the relay contacts by performing an operation that a current flows at the moment that the relay contact is closed, it is possible to expect an effect capable of suppressing the occurrence of the open failure of the relay contact.
As a method or an apparatus for solving such a problem, there is a control apparatus for electric power steering apparatus that is described in Japanese Published Unexamined Patent Application No. 2010-132206 A (Patent Document 2). That is to say, as shown in FIG. 5, the control apparatus of Patent Document 2 detects A-phase motor current by forcibly exciting an A-phase motor relay 42 and a B-phase motor relay 44 and, by simultaneously setting the duty ratio of the PWM signal PS for driving the switching elements of the motor drive circuit 106, to a specified duty ratio for an abnormal diagnosis. Then, the control apparatus determines whether the absolute value of the detected A-phase motor current is equal to or more than a given threshold or not, and determines a relay contact 42a of the A-phase motor relay 42 and a relay contact 44a of the B-phase motor relay 44 are normal or abnormal. In the case of determining that the relay contact is abnormal, repeating only a given determination times of the determination of normal/abnormal, and in the case of determining that the relay contact is abnormal even after performing the determination several times, confirming the abnormal determination and then performing a fail-safe processing.